The present invention relates to an improved process for the forming of coupling pieces from a thermoplastic, which coupling pieces are provided with at least one undercut annular groove, in which process the thermoplastic is shaped, by means of a forming core provided with an annular rib of which the external circumference corresponds to the shape which is to be imparted to the coupling piece, under temperature and pressure conditions which are such that the forming core is closely surrounded by the thermoplastic, and such that a coupling piece provided with an undercut annular groove is preformed, part of the coupling piece preformed in this way is cooled so that the narrowed terminal part of the coupling piece, extending from the free end of the coupling piece to the point of greater depth of the undercut, is kept at a temperature which is such that this part remains elastically deformable, the forming core, including the rib with which it is provided, is then removed from the coupling piece through the narrowed part by virtue of an elastic deformation of the latter, the elastically deformed terminal part is then cooled to a temperature which is such that the thermoplastic is no longer deformable, and, finally, the coupling piece formed in this way is completely separated from the forming core. The present invention also relates to a device suitable for carrying out this process.
A process of this type is described in German patent application No. 2,633,050 in the name of ANTON ANGER MASCHINENBAU GmbH. In accordance with this known process, in which the forming core is surrounded by the thermoplastic by injection moulding, the elastically deformed terminal part of the moulded coupling piece finally resumes its preformed shape, under the effect of its elasticity, after the forming sleeve has been withdrawn.
However, this known process exhibits the disadvantage that the precision of shape and dimensions of the deformed terminal part of the coupling piece produced in this way greatly depends on the temperature maintained in the thermoplastic, and also on the shape of the undercut, and is thus difficult to control. In practice, this known process is not in fact suitable as such for the production of coupling pieces for which a high precision of shape and dimensions is required.